virt_vm: Plug some more fd leaks

Close log files that are opened one time, such as the
ones opened for file transfers. This helps to reduce
even further the amount of files opened after a given
test job, specially if it involves a lot of file transfers.

With this patch, this test set:

./run -t qemu -g RHEL.6.3.x86_64 --nettype bridge --tests "migrate.with_autotest"

Comes from 94 files opened at the end to 43.

CC: Lei Yang <yanglei.fage@gmail.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>